1. Field of the Invention
The present invention relates generally to GPS processing and more particularly to GPS processing of GPS positioning signals in faded environments.
2. Description of the Related Art
There is a growing need for accessing GPS positioning signals in faded environments (e.g., indoors, below ground). The fades encountered can be more than 30 dB in some cases. The sensitivity of the GPS receiver must be increased commensurately to operate in these environments. Since high gain antennas cannot normally be used in these faded environments and lowering the noise figure can prove to be very expensive, the processing gain must be increased by increasing the integration time as much as possible. This requires that the phase profile of the near-baseband signal after GPS-message and code wipe-off will be matched. The linear part of the phase profile can be matched by trying multiple hypotheses, for phase rate, as with an FFT. With long coherent integration times, however, the nonlinear part of the phase profile will cause significant losses unless it is removed prior to inputting the signal to the FFT. The nonlinear part of the phase profile arises from several sources, the phase drift of the GPS satellite atomic clock, the phase drift due to the motion of the GPS satellite, the phase draft due to the motion of the GPS receiver and the phase drift due to the GPS receiver's clock. The non-linear phase caused by the phase drift of the GPS satellite's atomic clock is very small and does not become significant until the coherent integration time reaches about 10 seconds. For earth-fixed receivers, the nonlinear phase caused by the phase drift due to the motion of the GPS satellites is easy to remove given very approximate a-prior receiver position information. Several kilometers are adequate for integration times over one second. The requirement for this a-prior information can be eliminated by testing various hypotheses for receiver location. The nonlinear phase caused by phase drift due to the motion of the GPS receiver does not exist for receivers that are fixed or move at a constant velocity. There is usually no way to predict the nonlinear phase induced by the phase drift due to the GPS receiver's clock. The present invention addresses these concerns.